Nowadays, optical transmission lines convey signals which are wavelength multiplexed. These signals are amplified all along their transmission by optical fiber amplifiers.
The present trend is to make ever increasing use of optical solutions for performing all of the transmission over a transmission network.
In a transmission network, there are not only transmission functions proper, but also routing, configuration, or reconfiguration functions for conveying information to a given outlet point from the network.
Unfortunately, when transmission is performed over a network, heavy traffic or other reasons can make it necessary to reconfigure the network in appropriate locations, thereby changing the number of transmission channels which propagate over optical transmission lines and which are amplified by the optical fiber amplifiers all along said lines.
Optical fiber amplifiers, and more particularly erbium-doped fiber amplifiers, are used on optical transmission lines since they do not present gain non-linearity as a function of the power of the input signal at the modulation frequencies of the signals used in telecommunications systems.
The gain recovery time in an erbium-doped fiber amplifier is greater than 0.1 ms. This long recovery time serves to stabilize gain since gain does not have the time to rise when the signal passes from a high state to a low state at the modulation frequencies used in telecommunications which are of the order of 100 MHz to 10 GHz.
Unfortunately, it has been observed that when the number of transmission channels present at the input to an optical fiber amplifier is changed, that has the effect of saturating or desaturating the amplifier which leads to a transient phenomenon. The gain of the amplifier varies in transient manner and the total power in the output signal drops.
This phenomenon is troublesome since it means that for a very short length of time, typically several tens of microseconds, the power of the channels actually in use is changed and, unfortunately, that can lead to transmission errors.
To solve this problem, the state of the art proposes a system shown as a block diagram in FIG. 1.
The terminal T represents an transmitter or a routing node in the network, and terminal R represents a receiver or some other routing node in the network. Erbium-doped fiber amplifiers (EDFAS) are present in the receivers located all along the line between its access and outlet points T and R.
That system makes use of a laser source L(.lambda.c) placed at the input of line F and having output power that is servo-controlled by its current feed so that the total power of the useful signal plus the power of the signal emitted by the laser remains constant. For that purpose, a servo-control loop BA picks up a small fraction of the total power of the signals to enable detector means DP to detect the level of the total power transmitted over the line and to apply feedback to the current through the laser L.
The laser L is selected to operate at a wavelength .lambda.c that is different from the wavelengths of the channels used .lambda.1-.lambda.n.
Thus, supposing there are five channels and that for routing reasons three of the channels are removed, then the laser regulation loop will increase the output power of the laser so that the power of the two remaining channels plus that of the laser corresponds to the power of the five initial channels.
The drawback of that solution is to introduce an additional component which is a laser diode and a fast electronic feedback loop including a circuit for controlling the laser. That solution is relatively complex and expensive.